Visual acuity has been the single most important parameter for measuring performance of human vision for over 150 years because it is a signal number and measures the combined performance of optics, retina, and visual signal processing by the brain.
Visual acuity is however also limited for measuring quality of vision for a number of reasons. First, visual acuity is incomplete because it only measures vision performance for one particular resolution task. Excellent acuity does not guarantee excellent vision. It is possible that someone may have a visual acuity of 20/20 or better but has problems in reading low-contrast texts or has night vision symptoms like glare, halo, and ghost images. Second, visual acuity measures vision performance too coarsely. Among all eyes with the same acuity of 20/20, their true quality of vision can vary significantly from eye to eye. Visual acuity is thus not effective for specifying vision quality in fine details. Third, visual acuity does not measure vision in all lighting conditions. Visual acuity is usually measured in one pupil size clinically. Pupil diameters of eye are known to change significantly depending on the level of surrounding light and quality of vision is important for all pupil sizes.
In light of the limitations of visual acuity, it is readily apparent that a need exists in the art to provide methods for grading quality of vision in finer details than visual acuity, for measuring vision with more general tasks than visual resolution and at multiple pupil sizes.
One known method beyond visual acuity is to specify image quality of eye using Modulation Transfer Functions (MTF). MTF of eye can be obtained in a number of different means. One practical method is to calculate MTF of an eye from wave aberration as disclosed in “Aberrations and retinal image quality of the human eye,” J. Opt. Soc. Am. A, vol. 14, no. 11, p. 2873 (November 1997) by J. Liang et al. Wave aberrations are usually obtained from a device called aberrometers including ray tracing aberrometers, Talbot interferometry-based aberrometers, and phase retrieval method-based method, and the Hartmann-Shack sensor based-aberrometer.
FIG. 1 shows a schematic diagram for a typical wavefront system using a Hartmann-Shack sensor as disclosed in “Objective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensor,” J. Opt. Soc. Am. A, vol. 11, no. 7, p. 1949 (July 1994) by J. Liang et al. A fixation system (110) assists the tested eye in stabilizing its accommodation and in maintaining the view direction. An illumination light source (120) generates a compact light source to reflect off mirror (BS2) and shine on the eye's retina as the probing light. The probing light is diffusely reflected by the retina, from which a distorted wavefront is formed at the eye's cornea plane. An optical relay system (130), consisting of lenses (L1) and (L2), relays the outgoing wavefront from the eye and reflected off of mirror BS1 to the plane of a lenslet array. A Hartmann-Shack wavefront sensor (140), consisting of a lenslet array and an image sensor, produces a wavefront sensor image as an array of focus spots. An image analysis module (150) detects the focus spots and calculates the wavefront slopes, from which the wavefront is reconstructed by a wavefront estimator (160).
Wavefront system for the eye often measure wave aberration for a large pupil at very low light level. From wave aberration in a large pupil, optical quality of an eye can be calculated for any pupil size that is smaller than the measured pupil size. FIG. 2 shows radially averaged MTF of the eye for 6 different pupil sizes derived from wave aberrations for 14 eyes within a dilated 7.3 mm pupils as disclosed in “Aberrations and retinal image quality of the human eye,” J. Opt. Soc. Am. A, vol. 14, no. 11, p. 2873 (November 1997) by J. Liang et al.
Using MTF in its original form is not practical in clinical settings for at least two reasons. First, MTF is a scientific term and it specifies the transfer ratio of image contrast from the object space to the image space. Few clinicians would be able to interpret its clinical meaning in its original form. Second, there is so far neither an effective method nor an acceptable standard for grading eye's MTF clinically. As a common practice, MTF of different eyes are compared at the same pupil size for fairness. Such a fair comparison is however often meaningless because the same pupil size may be used in different vision conditions for different eyes. Vision at a 6 mm pupil size may represent day vision for one person with a large natural pupil and night vision for another. Additionally, comparing night vision for different eyes cannot be performed at the same pupil size because pupil sizes at night can vary significantly from eye to eye as illustrated in FIG. 3, showing images of three eyes at night. The natural pupil sizes of the three eyes are 4.7 mm (FIG. 3a), 6 mm (FIG. 3b), and 8.5 mm (FIG. 3c), respectively. It is not difficult to conclude that comparing MTF at the same pupil is useless for night vision.
In light of the forgoing, it will be readily apparent that a need exists in the art to provide a clinical MTF system that is understandable in clinical settings. More importantly, the clinical MTF system enables to specify quality of vision in a plurality of grades under same visual acuity. It is also apparent that a need exists in the art to provide a more effective method for comparing vision under equal conditions at different pupil sizes.